From: roberts%studguppy@LANL.GOV (Doug Roberts @ Los Alamos National Laboratory)
"5. Has this ever happened to anyone else? I racked my Clara Bell batch
"from the primary to the secondary day before yesterday. The head had
"just fallen after a healthy, vigorous initial fermentation. However, I
"noticed the next day that fermentation had completely stopped. I've
"experienced this before where racking seems to shock the yeast
"temporarily (up to a few days), and then fermentation resumes. I'm
"always careful to let the wort cascade down the side of the glass
"carboy to minimize oxygentation, and I always make sure to syphon a
"big slug of yeast with the wort in an attempt to assure continuous
"fermentation. However, about half the time I notice that
"fermentation comes to a complete halt for up to 3 or 4 days, after
"which it slowly resumes. Any ideas?

After the head falls, most of the yeast has run out of food and is falling
out of solution and becoming dormant. One of the reasons why you get a
head on fermenting beer is the same as why you get it on a finished beer--
there are CO2 bubbles building a head in conjunction with the protiens.
When the fermentation slows down, there is a lot less CO2 coming off the
beer, so there isn't anything to form a head with. Also, I usually notice
that the airlock doesn't move much after racking. I attribute this to
the fact that I have purged most of the CO2 out of solution as well, by
running it through that narrow racking tube. This is actually a Good
Thing (tm) because it means that I've purged the air out of the carboy
I'm racking into. But it takes the slowed down yeast quite a while to
once again reach a saturated solution of CO2; only after this will
the CO2 begin evolving from the beer again.